U.S. patent number 5,598,097 [Application Number 08/278,936] was granted by the patent office on 1997-01-28 for dielectric resonator-based electron paramagnetic resonance probe.
This patent grant is currently assigned to Research Foundation of State University of New York. Invention is credited to Charles P. Scholes, Andrzej Sienkiewicz.
United States Patent |
5,598,097 |
Scholes , et al. |
January 28, 1997 |
Dielectric resonator-based electron paramagnetic resonance
probe
Abstract
An apparatus for use in electron paramagnetic spectroscopy
having a resonant structure with one or more dielectric resonators,
the resonant frequency of which may be varied by varying the
distance between the dielectric resonators, the coupling between
the coupler and resonant structure may be varied by a remote
coupling matching device without changing the resonant frequency of
the resonant structure.
Inventors: |
Scholes; Charles P. (Delmar,
NY), Sienkiewicz; Andrzej (Albany, NY) |
Assignee: |
Research Foundation of State
University of New York (Albany, NY)
|
Family
ID: |
23067019 |
Appl.
No.: |
08/278,936 |
Filed: |
July 22, 1994 |
Current U.S.
Class: |
324/316;
324/318 |
Current CPC
Class: |
G01R
33/345 (20130101); G01R 33/60 (20130101) |
Current International
Class: |
G01R
33/34 (20060101); G01R 33/60 (20060101); G01R
33/345 (20060101); G01V 003/00 () |
Field of
Search: |
;324/316,318,322,300,301,307,309 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Walsh, Jr. et al. "Enhanced ESR sensitivity using a dielectric
resonator" Rev. Sci. Instrum. 57, 2278-2279 (1986). .
Kajfez et al. "Computed Modal Field Distribusions for Isolated
Dielectric Resonators" IEEE Trans. on Micro. Theory and Tech.
MTT-32, 1609-1616 (1984). .
Jaworski et al. "An Accurate Solution of the Cylindrical Dielectric
Resonator Problem" IEEE Trans. on Micro. Theory and Tech. MTT-27,
639-643 (1979). .
Fiedziuszko et al. "The Influence of Conducting Walls on Resonant
Frequencies of the Dielectric Microwave Resonator" IEEE Trans. on
Micro. Theory and Tech. MTT-13, 778-779 (1971). .
Fiedziuszko et al. "Double Dielectric Resonator" IEEE Trans. on
Micro. Theory and Tech. MTT-13, 779-781 (1971). .
Dykstra et al. "A Dielectric Sample Resonator Design for Enhanced
Sensitivity of EPR Spectroscopy" Journal of Magnetic Resonance 69,
350-355 (1986). .
Bromberg et al. "Enhanced sensitivity for high-pressure EPR using
dielectric resonators" Rev. Sci. Instrum. 63, 3670-3673 (1992).
.
Hubbell et al. "Continuous and stopped flow EPR spectrometer based
on a loop gap resonator" Rev. Sci. Instrum. 58, 1879-1886
(1987)..
|
Primary Examiner: Arana; Louis M.
Attorney, Agent or Firm: Heslin & Rothenberg, P.C.
Government Interests
The United States Government has a paid up non-exclusive license in
this invention as provided by the terms of Grant No. 5R016M3510320
awarded by the National Institute of Health (NIH).
Claims
What is claimed is:
1. An electron paramagnetic resonance probe comprising:
at least one dielectric resonator having a hole extending axially
therethrough;
a microwave shield surrounding said resonator, said microwave
shield having a hole extending axially therethrough and comprising
a low loss dielectric polymeric material having a metallic coating
wherein the hole within said microwave shield is aligned with the
hole of said at least one dielectric resonator; and
a coupler wherein microwaves generated from a distant source are
coupled to said resonator.
2. An electron paramagnetic resonance probe of claim 1 wherein said
at least one dielectric resonator comprises a single cylindrical
resonator.
3. An electron paramagnetic resonance probe of claim 2 wherein said
coupler comprises:
a microwave transmission line; and
an antenna wherein said antenna is disposed adjacent said single
dielectric resonator.
4. An electron paramagnetic resonance probe of claim 1 wherein said
at least one dielectric resonator comprises a first and second
dielectric resonator having a hole extending axially therethrough,
said second resonator being disposed above said first resonator
wherein the hole within said first and second dielectric resonators
are aligned.
5. An electron paramagnetic resonance probe of claim 4 wherein said
coupler comprises:
a microwave transmission line; and
an antenna, wherein said antenna is disposed adjacent and
substantially equidistant from said first and second
resonators.
6. An electron paramagnetic resonance probe of claim 5 wherein said
at least one dielectric resonator comprises a first and second
dielectric resonator and wherein a low loss dielectric spacer is
disposed between said dielectric resonators.
7. An electron paramagnetic resonance probe of claim 4 wherein said
dielectric resonators are disc shaped.
8. An electron paramagnetic resonance probe of claim 7 wherein said
microwave shield conforms to surround the outer perimeter of said
at least one dielectric resonator.
9. An electron paramagnetic resonance probe of claim 1 wherein said
at least one dielectric resonator comprises first and second
dielectric resonators separated by a predetermined distance.
10. An electron paramagnetic resonance probe of claim 1 wherein
said microwave shield comprises a low loss dielectric material
having an outer surface coated with a metallic paint.
11. An electron paramagnetic resonance probe of claim 10 further
comprising an outer body surrounding said microwave shield adapted
to form a connection with a mixer housing.
12. An electron paramagnetic resonance probe of claim 10 further
comprising an outer body surrounding said microwave shield having a
hole extending axially therethrough and a capillary adapter.
13. An electron paramagnetic resonance probe of claim 1
wherein:
said at least one dielectric resonator is cylindrically shaped;
said microwave shield comprises a cylindrical side wall, the outer
portion of said side wall having a coating of a metallic paint; and
further comprising first and second lids sized to fit within said
cylindrical side wall adjacent said at least one dielectric
resonator;
said first and second lids having a hole extending through the
length of said lids wherein when said first and second lids are
placed within said cylindrical side wall, and wherein the hole
within said at least one dielectric resonator is aligned with the
holes of said first and second lids.
14. An electron paramagnetic resonance probe of claim 13 further
comprising a metallic sleeve disposed within the hole of said first
lid.
15. An electron paramagnetic resonance probe of claim 1 wherein
said coupler comprises a microwave transmission line and a
non-linear shaped antenna lying within a plane perpendicular to an
axis extending through said aligned holes, said antenna being
placed near said at least one dielectric resonator.
16. An electron paramagnetic resonance probe of claim 15 further
comprising a coupling matching device having a center conductor
wherein said antenna, center conductor and microwave transmission
line are electrically connected.
17. An electron paramagnetic resonance probe of claim 16
wherein:
said center conductor extends into a tube; and
a moveable metallic plunger sized to fit within said tube and
having an opening extending axially through said plunger sized to
accept said center conductor wherein the plunger may be moved
axially within said tube thereby changing the electrical length of
the coupling matching device.
18. An electron paramagnetic resonance probe of claim 15 wherein
said non-linear shaped antenna is substantially hook shaped.
19. An electron paramagnetic resonance probe of claim 1 wherein
said coupler comprises a microwave transmission line and a
non-linear shaped antenna within a plane parallel to an axis
extending through said aligned holes, said antenna being placed
near said at least one dielectric resonator.
20. An electron paramagnetic resonance probe of claim 1 further
comprising
a remote coupling matching device operatively engaged to said
coupler whereby the resonant frequency remains substantially
unchanged upon modification of the coupling between the resonant
structure and the coupler.
21. An electron paramagnetic resonance probe of claim 20 wherein
said coupling matching device comprises a coaxial waveguide of
adjustable length.
22. An electron paramagnetic resonance probe of claim 21 wherein
said coupler comprises an antenna, connector and microwave feed
line.
23. An electron paramagnetic resonance probe of claim 22 wherein
said connector comprises a three-way connection between said
conductor, said waveguide and said antenna.
24. An electron paramagnetic resonance probe of claim 21 wherein
said coaxial waveguide of adjustable length comprises:
a hollow tube;
a conductor disposed in said tube, electrically connected to said
coupler; and
a metallic plunger surrounding said cable and axially moveable
along the length of said cable.
25. An electron paramagnetic probe of claim 24 wherein said coupler
comprises:
an antenna comprising an inner conductor of coaxial cable; and
a microwave feed line, a portion of which comprises coaxial cable
having an inner and outer conductor wherein the inner conductor of
the coaxial cable of said antenna, waveguide and microwave feed
line are electrically connected to form a three-arm connection.
26. An electron paramagnetic resonance probe of claim 24 wherein
said resonant structure comprises a resonator surrounded by a
microwave shield.
27. An electron paramagnetic resonance probe of claim 24 further
comprising: a mixer assembly in connection with a capillary,
wherein said capillary extends through said resonant structure.
28. An electron paramagnetic resonance probe of claim 20 further
comprising a protective body surrounding said resonant structure.
Description
TECHNICAL FIELD
The present invention relates to an apparatus for practicing
electron paramagnetic resonance spectroscopy, more particularly, to
a dielectric resonator-based electron paramagnetic resonance probe
having a tunable coupler.
BACKGROUND OF THE INVENTION
Electron paramagnetic resonance (EPR) has been used to make direct
measurements of reaction kinetics, particularly for those reactions
involving free radicals. Exemplary applications of stopped-flow EPR
include studies of: the kinetics of enzymatic molecular oxygen
consumption; kinetics of micromolar quantities of spin-trapped free
radicals followed on a sub-second time scale; time resolved protein
folding and unfolding; and oxygenation of various organic molecules
as performed in research concerning the effects of free radicals in
ischemia.
Many EPR spectrometers utilize a conventional cavity resonator
similar to one described in Yamazaki et al., J. Biol. Chem., 235,
2444 (1960). These resonators are characterized generally by a
rectangular metallic structure or frame, the inside of which is a
resonant cavity through which a capillary or flat cell may extend.
These conventional cavity resonators are typically ill suited for
the study of lossy dielectric samples, which includes most
biologicals and solutions of free radicals. The sample volumes
utilized by the conventional cavity resonators are measured by the
milliliter. However, biological samples are often limited in supply
which presents a particularly troublesome problem since employing a
conventional cavity resonator to study transient processes usually
requires large volumes of relatively concentrated material. Another
problem arises from the failure of the conventional cavity
resonator to effectively isolate the region of microwave electric
field (E.sub.1) from the region of microwave magnetic fields
(H.sub.1), the latter of which induces the desired EPR transitions.
The inability to separate the E.sub.1 and H.sub.1 components is an
important characteristic since the electric field may often
interact with a sample to cause resonant frequency changes and Q
losses (Q is the quality factor, either calculated as being
2.pi..times.microwave energy stored by the device/energy dissipated
per cycle of microwaves or calculated as the resonant frequency
(.nu..sub.o) of the device/the difference in frequency
(.DELTA..nu.) obtained at the 3 dB half power absorbing points on
the mode pattern of the device). This undesirable interaction
between the sample and the E.sub.1 component is especially
pronounced with lossy dielectric samples.
A design more recently used today for continuous and stopped flow
EPR is based on a loop gap resonator (LGR) as described in Hubbell
et al. Rev. Sci. Instrumen., 58, 1879 (1987). The standard design
for an LGR utilizes a machined MACOR.RTM. ceramic block having two
holes extending through the block, these holes are connected by a
thin slit extending through said block, the interior of the holes
and slit are plated with silver. Unlike the conventional cavity
resonators the LGR utilizes a much smaller sample volume, however,
due to the complex configuration of the LGR and its small
components the LGR based EPR probe is typically susceptible to a
significant loss of sensitivity with use. In addition, due to the
configuration of the loop and gap areas of the LGR low Q is
experienced due to electric field (E.sub.1) interaction with lossy
dielectric samples. In addition, due to the design of the LGR, flow
and stopped-flow induced noise is a limiting factor when utilizing
stopped flow technology since repetitive starting and stopping of
the sample flow in the capillary is required. This forced movement
within the capillary tube creates vibrations which effectively
limit the sensitivity of the LGR. In addition, the structure of the
LGR makes it difficult to assemble and disassemble the device. In
the event any particular part becomes contaminated or worn, the
ability to replace or repair any individual component takes
considerable effort and often requires returning the part to the
manufacturer. In addition, the use of delicate and complex machined
parts results not only in less durable parts but in expensive
replacement parts. Furthermore, variable capacitance coupling used
in connection with the LGR probe often causes large resonance
frequency changes when the coupling is changed. The resulting
simultaneous coupling and frequency changes greatly complicate
attaining critical coupling.
Many EPR spectroscopy systems, such as existing Bruker systems,
which lack the GaAsFET amplifier often have significant difficulty
in making their AFC (automatic frequency control) lock to a low Q
resonator. This difficulty is more commonly experienced at low
powers. Difficulty in obtaining an AFC lock may cause frequency
drift, drift in AFC error voltage, uncertain admixture of
absorption and dispersion and noise. A higher Q system makes it
easier to obtain an AFC lock without GaAsFET amplifications.
Therefore there exists a need for an EPR probe having a high Q, in
particular, one suitable for use in the study of highly lossy
samples and that is also capable of maintaining a high Q value when
studying such samples. In addition, there exists a need for an EPR
probe which utilizes small sample volumes but is capable of
withstanding the vibrations created by stopped flow techniques and
capable of maintaining a high Q and its sensitivity. In addition,
there exists a need for an EPR probe having a simple and durable
design which is readily assembled and disassembled and capable of
quick and inexpensive repair.
SUMMARY OF THE INVENTION
The shortcomings of the prior art are overcome and the
aforementioned objects are achieved in accordance with the
principles of the present invention, which may comprise an electron
paramagnetic resonance probe having at least one dielectric
resonator with a hole extending axially therethrough; the
resonators are surrounded by a microwave shield also having a hole
extending axially therethrough such that a capillary may extend
through both the microwave shield and the one or more dielectric
resonators; and a coupler wherein microwaves generated from a
distant source are coupled to said resonator. The electron
paramagnetic resonance probe may comprise a single cylindrical
dielectric resonator or two or more cylindrical dielectric
resonators disposed vertically in relation to one another and with
each having a hole extending axially therethrough wherein a
capillary tube may extend through the hole within said first and
second dielectric resonators. A low loss dielectric spacer may be
disposed between multiple dielectric resonators. The dielectric
resonators may comprise a highly dielectric, but non-lossy,
material.
The coupler of the present invention may comprise: a microwave
transmission line; and an antenna, wherein said antenna is disposed
adjacent the spacer and substantially equidistant from the first
and second resonators. When a single resonator is utilized the
antenna may be disposed adjacent to the single dielectric
resonator. The coupler may further comprise a microwave
transmission line and a non-linearly shaped antenna within a plane
perpendicular to the axis of said capillary.
The electron paramagnetic resonator may further comprise a coupling
matching device having a center conductor connected to an antenna.
The center conductor and microwave transmission line are
electrically connected. The center conductor may extend into a
metal sleeve in which a movable metallic plunger, sized to fit
within the sleeve and having an opening extending axially through
said plunger sized to accept the center conductor, may be moved
axially within the sleeve thereby changing the electrical length of
the coupling matching device.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partially cutaway side view of the dielectric
resonator-based EPR probe.
FIG. 2 is a partially cutaway side view of the resonator chamber,
protective body and SMA connector.
FIG. 2B is a top cross-sectional view of FIG. 1 taken from B, B'
for the TE mode.
FIG. 2C is a top cross-sectional view of FIG. 1 taken from B, B'
for the TM mode.
FIG. 3 is a cross-sectional side view of the resonant structure
having two stacked dielectric resonators and antenna.
FIG. 3B is a cross-sectional view of the resonant structure having
a single dielectric resonator and antenna.
FIG. 4 is a partially cutaway front view of the stopped-flow EPR
probe and coupler.
FIG. 4B is a cross-sectional view of the apparatus taken from A,
A.sup.1 of FIG. 4.
FIG. 5 is a graph displaying resonant frequency as a function of
spacing between two stacked dielectric resonators.
DETAILED DESCRIPTION
The present invention relates to a durable and efficient dielectric
resonator based EPR probe. In accordance with the invention the EPR
probe may include a resonant structure, a coupler and a coupling
matching device.
As can be seen in reference to the embodiment shown in FIG. 1, the
resonant structure may include a pair of dielectric resonators 20
separated by a low loss dielectric spacer 22 contained in a
microwave shield 30 having a metallic covering 38. Surrounding the
resonant structure may be a protective body 40 capable of accepting
a mixer assembly 70 and capillary adapter 54. The resonant
structure and protective body are designed such that a capillary 10
may extend from the mixer assembly 70 through the protective body
40 and resonant structure to the capillary adapter 54.
Extending from within the body of the microwave shield 30 of the
resonant structure is an antenna 92. The antenna is a portion of
the coupler which may comprise a microwave transmission line 82
connected to a microwave production source (not shown) and a
connector 84. The antenna 92 protrudes laterally into the microwave
shield 30 adjacent to the dielectric resonator 20 or spacer 22. The
microwave transmission line 82 extends to and within the connector
84 which electrically connects the incoming microwave transmission
line 82, semi-rigid coaxial cable 90 (which is connected to the
antenna 92) and the center conductor 102 (which is part of the
coupling matching device 100). The coupling matching device 100 may
comprise a center conductor 102 and a tightly fitting metal plunger
104 surrounding a portion of the center conductor 102 both of which
extend within tubing 106. The metal plunger 104 is capable of being
moved back and forth along the length of the center conductor
102.
As can be seen in FIGS. 3 and 3B the resonant structure may utilize
one or more dielectric resonators 20, an optional spacer 22 and a
microwave shield 30. The dielectric resonator 20 may be toroidal,
cylindrical or rectangular in shape having a hole extending axially
through the center. Preferably the dielectric resonator 20 is
substantially disc shaped, the dimensions will vary as a function
of the dielectric constant (.epsilon.) of the material comprising
the dielectric resonator and also upon the resonant frequency of
the resonant structure. For X-band wavelengths with ceramics having
a (.epsilon.).congruent.30, the dimensions preferably include a
dielectric resonator having an outer diameter (o.d.) 6.00 to 6.55
mm, inner diameter (i.d.) 1.00 to 3.00 mm and height 2.25 to 2.95
mm. The i.d. of the resonator should be large enough to allow the
capillary 10 (FIG. 2) to extend therethrough. A common capillary
used is fused quartz tubing 0.84 mm o.d., 0.60 mm i.d. and 100 mm
in length. The dielectric resonator 20 is preferably comprised of a
highly dielectric material having a dielectric constant greater
than 10, with a preferred dielectric constant (.epsilon.) greater
than approximately 30, and a dielectric loss less than
1.times.10.sup.-4, preferably under 8.0.times.10.sup.-5. Examples
of such dielectric materials include titanium oxide ceramics,
examples being (ZrSn)Ti Oxide; BaLaTi Oxide; BaZnTaTi Oxide; Ba Ti
Oxide. A preferred dielectric resonator is manufactured by
MuRata-Erie North America, Inc., of State College, Pa., part number
DRT060SO20CO27AM and Transtech Inc. of Adamstown, Md., part number
C8733-0245-X-110-B-040. However, any appropriately sized and shaped
dielectric material of low dielectric loss and containing no
paramagnetic centers may function as a resonator in the present
invention.
When multiple dielectric resonators 20 are utilized they may be
separated by a low loss dielectric spacer 22 having a low
dielectric constant (.epsilon.). Preferably the spacer is comprised
of a material having a .epsilon.<3. Examples of such materials
include, polystyrene, Rexolite.RTM., Teflon.RTM., KelF.RTM..
Rexolite.RTM. 1422 plastic manufacture by C-LEC Plastic, Inc. of
Beverly, N.J. is a preferred material due to its low cost, easy
machinability and good dielectric characteristics (no EPR
background, a low dielectric loss of 6.6.times.10.sup.-4 and a low
dielectric constant of 2.53 at 10 GHz). Inexpensive and easily
machinable products are preferred for use in the present invention
as the specific size and shape of the individual spacers will vary
with the desired resonant frequency (f.sub.o). Preferably the
spacer 22 is shaped to have the same outer and inner diameter of
the dielectric resonators it is placed between, however, the
preferred length will ultimately be dictated by the desired
f.sub.o.
Surrounding the top, bottom and sides of the dielectric resonator
20 and the optional spacer 22 is a microwave shield 30, which
together with the resonators 20 comprise the resonant structure.
The microwave shield 30 also prevents unwanted radiation losses and
noise from the dielectric resonators and active volume of the
sample capillary. The microwave shield 30 may comprise a body of
low loss dielectric material, such as that utilized in the spacer.
The low loss dielectric body preferably has a thickness of at least
10 mm. The outer surface of the microwave shield 30 has a metallic
covering 38 capable of acting as a shield. Examples of such
metallic coatings include: silver paint, gold paint, copper
coatings including copper foil; copper, silver or gold electrolytic
plating.
As can be seen in FIGS. 2 and 3, one embodiment of the microwave
shield 30 may comprise low loss dielectric lids 34 sized to have
the same outer and inner diameter of the dielectric resonators 20
and shaped to lie flat upon the resonators 20. These lids 34 are
placed on the top and bottom portions of the dielectric resonator
20 (FIG. 3) or the exposed top and bottom portions opposite the
spacer 22 (FIG. 2). The dielectric resonators 20, the spacer 22 and
top and bottom lids 34 are contained within the microwave shield 30
which comprises a cylindrical side wall having an opening extending
axially therethrough and sized to accept these components. The
cylindrical side wall, 3.34 mm thick as show in FIG. 1, has an
opening adjacent to a single resonator 20 (FIG. 3B) or spacer 22
(FIG. 3) large enough to accept the antenna 92 and has an outer
surface coated with 0.05 mm of a silver coating such as DuPont.RTM.
silver paint. Copper foil 50 (FIG. 2) is placed along the top and
bottom of the cylindrical side wall and lids.
As can be seen in FIGS. 1, and 4B, the resonant structure may be
substantially surrounded by a protective body 40 which acts to
protect the shield 30 and its metallic covering 38 from damage or
contamination. A rigid and durable material should be used in the
protective body, examples of such materials include plastics such
as those sold under the trade names Delrin.RTM., Acido.RTM. and
Poylpenco.RTM.. As can be seen in reference to FIG. 1, the
protective body 40 may comprise a durable body that fully encloses
the microwave shield 30 with the exception of an opening which
allows the capillary 10 and the antenna 92 to extend therethrough.
This protective body 40, in connection with an external bracket 42,
may also function as a bracket to which other elements, such as the
coupler and coupling matching device 100 may be attached.
In reference to FIG. 1, one embodiment of the protective body 40
may include a hollow cylindrical body with the microwave shield 30
disposed therein, an upper collar 44 and lower collar 46, both of
which extend into the interior of cylindrical body and lie on said
microwave shield 30. The cylindrical body is sized to have an i.d.
slightly larger than that of the microwave shield 30 and a length
which allows the microwave shield 30 to completely fit within the
interior of the protective cylindrical body. For example, in the
particular embodiment shown in FIG. 1, the outer cylindrical body
may be 25.4 mm in diameter and 26 mm long. The upper collar 44 is
sized to have a diameter substantially equivalent to the
cylindrical body with an extension of smaller diameter designed to
extend within the cylindrical body and lie on the microwave shield
30. The surface of the upper collar extension may have an
additional metallic covering 50, such as 0.1 mm thick copper foil
50 (FIG. 2), to provide additional microwave shielding. To further
improve the electrical contact of the shielding thin metal gaskets
51, approximately 0.05 mm thick, may be placed between the
microwave shield 30 and the metallic foil 50 adjacent the upper
collar 44. A brass sleeve 52 may likewise be inserted within the
end of the upper lid 44 which lies upon the microwave shield 30 to
prevent microwave leaks along the tube direction and also to
achieve better sample tube guidance. The capillary 10 will extend
through the brass sleeve 52 into the upper collar 44 and, thus, the
brass sleeve 52 must have an i.d. sufficient to accept the sample
capillary, for example one having a 3 mm o.d., 1 mm i.d., and 4.8
mm long. The upper collar 44 likewise has an opening extending
axially through the collar sized to allow the capillary to fully
extend therethrough. The upper collar 44 is preferably elongated
along the direction of the capillary to accommodate and protect the
portion of the sample capillary 10 which protrudes from the
microwave shield. This particular design may accommodate 100 mm
standard length sample tubes or capillaries.
The portion of the upper collar 44 opposite the end in contact with
the microwave shield 30 may provide a seat for a chemically
resistant capillary adapter 54. The upper portion of the upper
collar may assume any one of numerous configurations which allow
the formation of a liquid tight seal between the capillary and
other means commonly used to collect capillary outflow. For
example, as shown in FIG. 1, the sample capillary 10 may be held in
a fixed position within the resonant structure by compression
O-ring seals 56, 76. An O-ring compression seal may be used to
create a seal between the outlet of the sample capillary 10 and the
termination of the chemically resistant exhaust hose 58, such as a
hose made of Polyether Ketone (PEEK). A small chemically resistant
compression fitting 60 may be inserted between the termination of
the exhaust hose 58 and the capillary adapter 54. The threaded
insert 62 of the exhaust hose 58 may be screwed onto the capillary
adapter 54, whereby the compression fitting 60 compresses the small
O-ring 56 (size 001) making a seal around the sample capillary 10.
The exact configuration of the upper lid may be altered or designed
in accordance with the chosen capillary and "work" space
available.
The lower collar 46 is similar to the upper collar 44 in that it is
also designed to have a diameter similar to that of the cylindrical
protective body and to have an extension of smaller diameter
extending within the cylindrical body to the microwave shield 30
such that the microwave shield 30 lies upon the lower collar
extension when within the cylindrical body. The lower collar 46
likewise has an opening extending vertically through the body of
the lid and a brass sleeve 52 such that a capillary 10 may extend
therethrough into the microwave shield 30. In addition, copper foil
50 may be placed upon the extension of the lower collar and a
silver gasket 51 upon the foil so that the microwave shield will
rest upon the silver gasket 51. A brass sleeve 53 may also be
inserted within the extension of the portion lower collar 46 upon
which the microwave shield 30 will lie and such that the capillary
10 may extend therethrough.
The lower collar 46 may also be modified to mate with a mixer
assembly 70 in a manner such that the mixer housing 72 is tightly
clamped and sealed to the lower collar 46. Any one of numerous
mixers and mixer housings may be utilized in conjunction with the
EPR probe of the present invention. Preferably they are made out of
a chemical resistant material such as polyester ketone (PEEK). A
preferred mixer is the Wiskind Grid Mixer manufactured by Update
Instrument of Madison, Wis. This particular mixer housing contains
4 metal grids (mesh size No. 75) and is centrally positioned
between the lower collar 46 and the mixer support 74. In the
embodiment shown in FIG. 1 the length of the brass sleeve 53 within
the lower collar 46 was changed to 3.5 mm to shorten the distance
between the outlet of the mixer assembly 70 and the center of the
active volume of the resonator. The bottom part of the lower collar
(opposite to the resonant structure) has a cylindrical hole which
accommodates the upper part of the mixer housing 72 and a small
O-ring 76 (size 001). This O-ring 76 seals the outlet of the mixer
and creates a seal around the sample capillary 10. The lower part
of the mixer housing 72 may be placed in the cylindrically shaped
seat of the mixer support 74, a larger O-ring 78 (size 007) may be
used to seal the lower part of the mixer housing 72 to the mixer
support 74. Two brass screws may be used to fasten the mixer
support 74 to the lower collar 46. As the mixer support 74 is
fastened to the lower collar 46, the two O-rings above and below
the mixer housing. 72 compress thereby sealing the mixer assembly
70. However, one skilled in the art may adapt the lower collar to
accommodate any one of numerous mixer assemblies. Chemically inert
hoses with threaded terminations and O-ring compression seals may
be used to connect the mixer support 74 to the one or more syringe
inlets 79. When utilizing two inlets, the two hoses may be oriented
in the horizontal plane and the outlets may form an initial "T-Jet"
injector inside the mixer support 74. The distance from the outlet
of the mixer to the center of the resonator is 12.6 mm within the
embodiment of FIG. 1. This distance, which contributes to the total
dead volume, can be slightly shortened by one skilled in the art
with other appropriate modifications. Standard EPR techniques,
other than stopped-flow, do not require a mixer assembly.
Delivery of liquid samples may be monitored by any one of numerous
systems known in the art, examples being a controlled syringe ram
or infusion pump, compressed air driven systems and hand driven
syringes. A preferred syringe ram system is the model 715 syringe
ram controller manufactured by Update Instruments, Inc. of Madison,
Wis.
As best seen in FIG. 2, microwaves are coupled to the resonant
structure by the coupler which may comprise an antenna 92, a
connector 84, a microwave transmission line 82 and a microwave
production source (not shown). The microwaves are directly coupled
to the resonant structure via the antenna 92, which extends within
the body of the microwave shield 30 adjacent, but not in contact
with, either the single resonator 20 (FIG. 3) or spacer 22 (FIG.
3B). The antenna 92 may comprise an extension of the center
conductor of a section of semirigid coax 90 which is in turn
attached to the center conductor of connector 84. The coupling loop
or antenna 92, which is preferably shaped to have a non-linear
configuration limited to a single plane, an example being a hook
shape (FIG. 2B) having approximately a 3 mm diameter made from 0.6
mm copper wire. The antenna is soldered to both the center
conductor and to the copper outer conductor of a 0.141 inch
semi-rigid coax 90. The antenna 92 protrudes laterally into the
microwave shield 30, preferably through a cylindrical hole (3.6 mm
in diameter) drilled in the shield 30 side wall. The position of
the antenna may be fixed adjacent to the dielectric resonator 20 or
spacer 22, and preferably with its plane oriented perpendicular to
the cylindrical axis of the resonant structure (FIG. 2B). However,
affixing the antenna in a plane parallel to the cylindrical axis of
the resonant structure (FIG. 2C) may also have useful applications.
Fixing the plane perpendicular to the cylindrical axis of the
resonant structure acts to couple the magnetic component (H.sub.1)
of the resonator TE.sub.01.delta. mode whereas orienting the plane
of the loop parallel to the cylindrical axis would preferentially
excite TM modes. A brass antenna guide 94, having an inner diameter
of 4.55 mm, may be used to guide the cylindrical part of the SMA
connector and tightly fit the termination of the semi-rigid coax 90
which leads from the SMA connector 84 to the antenna 92. The end of
the brass guide 94 for the cylindrical part of the SMA connector in
contact with the microwave shield 30 is preferably concave to
conform to the side of the shield 30. It is important to have good
electrical contact where the coupler joins the resonant structure.
Thus, a silver gasket 93 (FIG. 2) may be placed between the coated
wall of the microwave shield 30 and the brass guide 94. This silver
gasket 93 helps to prevent Q loss and to shield a point where poor
electrical contact can lead to microphonics.
Microwaves produced at a distant source (not shown), such as a
microwave bridge of an EPR spectrometer, propagate down a microwave
transmission line 82 to the connector 84. The transmission line 82
may comprise any suitable material known in the art to reliably
propagate microwaves of the selected wavelength, for example when
using X-band wavelength, the waveguide may be standard 0.141 inch
coaxial cable having an inner conductor of silver plated copper
wire, and an outer conductor of copper. As can best be seen in FIG.
1, the microwave transmission line 82 may be connected to a
connector 84, an example being a modified SMA (Standard Military A)
connector such as the Omni Spectra right angle cable plug, part
number 2007-5054-00. The inner conductor of the microwave connector
84, the inner conductor of semi-rigid coax 90 and center conductor
102 of the coupling matching device 100 are electrically
inter-connected within connector 84, preferably forming a
"T"-shaped joint. It is important to form a good electrical
connection between the inner conductor of the semi-rigid coax
leading to the coupling loop and the portion of the center
conductor in contact with the T-joint. In order to precisely orient
this electrical connection a technological hole may be drilled in
the back wall of the SMA connector to aid in aligning the three
conductors during attachment. This technological hole may be filled
once the connection is complete. An external bracket 42 (FIG. 1)
may be used to affix the coupler in position adjacent the resonant
structure as well as to compress the silver gasket 93 between the
microwave shield 30 and the brass antenna guide 94.
The coupling matching device 100 is connected to the coupler 80 at
the bottom wall of the connector 84. The center conductor 102,
which is the inner conductor of the coaxial line 106, extends from
the connector 84 into metallic tubing 106. Within the metallic
sleeve is a metallic plunger 104, both of which are preferably made
of a non-magnetic metal such as brass, which is sized to tightly
fit over the center conductor 102. The plunger is designed to move
back and forth within the sleeve 106 along the length of the center
conductor 102. The movement of the plunger, thus, modifies the
coupling between the resonant structure and coupler by changing the
electrical length of the coaxial coupling matching device.
Modifying the coupling, by matching the impedance between the
antenna and the microwave line allows the optional power to be
introduced into the resonant structure without significantly
changing f.sub.o of the resonant structure. The preferred length of
the plunger displacement is approximately 25 mm which roughly
corresponds to 3/4 .lambda. distance of the transverse
electromagnetic (TEM) line. This will typically cover the whole
tuning range needed to match impedance when using either high
dielectric lossy or standard non-lossy samples. Numerous mechanical
means exist for displacing the plunger as shown in FIG. 4, an
example being a gear box 108 connected to the plunger 104,
alternate means capable of moving a metal plunger are well known in
the art.
Having the entire coupling matching device 100 external to the
resonator allows the use of a simple and mechanically stable means
for changing the coupling. Stopped flow experiments involve rapid
sample movement through the Sample capillary and the rapid starting
and stopping of this flow causes concomitant vibrations. Thus, the
coupling and matching structures are preferably remote from the
sample capillary to avoid vibration-induced noisy transients. This
particular version of the variable short provides a smooth,
microphonic free operation and adds sufficient dynamic range to
relatively easily match various coupling with different loads.
Moreover, it allows one to change the coupling without causing
large resonance frequency shifts, separation of coupling and
frequency changes means that attaining critical coupling is
considerably simplified.
As can be seen in FIGS. 4 and 4A, stainless steel tubings 82 and 83
anchored firmly to bracket 42 enables the EPR probe to be attached
to the microwave waveguide output of the microwave bridge. One of
the stainless steel tubings may serve as guidance and protection
for driving shaft 81 of the coupling matching device 100. The
actual microwave connection to the waveguide output of the
microwave bridge may be achieved by a standard waveguide-to-coax
adapter which connects directly to the coaxial transmission line 82
and a section of the regular WR90 X-Band waveguide.
Since the DR-based EPR probe for stopped flow measurements directly
replaces the standard X-Band cavity of the conventional EPR
spectrometers, the present invention may easily be combined with
existing systems. No significant modifications are required to the
microwave bridge nor in the signal detection aspect of the
conventional EPR spectrometer.
The use of multiple dielectric resonators 20 will significantly
lower the resonance frequency and, thus, the resonant frequency may
tuned by varying the degree of separation between the stacked
dielectric resonators 20. For example, a Bruker ER-200 D EPR
spectrometer provided frequencies in a 9.05-10.0 Ghz region whereas
often a single dielectric resonator resonates above 10.0 Ghz or
else a double stacked pair resonates below 9.0 GHz. However,
increasing the distance between the stacked MuRata-Erie dielectric
resonators from 0.0 to 1.3 mm increased the resonant frequency in
the desired transverse electric (TE.sub.01.delta.) mode from 8.5
GHz to 9.21 GHz. Variation in resonant frequency (f.sub.o) in
relation to the spacing between two stacked TransTech dielectric
resonators can be seen in reference to FIG. 5. Thus, varying the
spacing between the dielectric resonators offers a straight forward
method of tuning that extends the active length of the dielectric
resonator device within the constrains of keeping the active volume
for the sample small. However, having a low loss dielectric between
the dielectric resonators suppresses transverse magnetic (TM) modes
at the expense of the desired TE.sub.01.delta. mode. The preferred
compromise of homogeneity of H.sub.1 across the sample and a
reasonable 0.7 cm active length utilizes the double stacked
dielectric resonator configuration having a separation of 1.3 mm to
1.5 mm.
Using a standard microwave sweeper, such as the Microline swept
oscillator Model 64X1, measurements of the resonant frequency
(f.sub.o), unloaded Q and loaded Q of the various DR configurations
may be made. This will allow calculation of the desired distance
between the stacked resonators. Thereafter, the resonant structure
and surrounding bodies may be easily machined, pieced together and
inserted into the apparatus. Thereafter, the microwave generator is
turned on and the sample inserted into the capillary via the
syringe inlet and the mixer. Once the sample is in the active
volume the coaxial coupling matching device may be adjusted to
obtain critical coupling. Movement of the Update RAM forces sample
from syringe to hose to mixer and to the capillary in the active
volume of the resonant structure. Then the EPR spectrometer
receives the EPR signal from the sample.
Due the use of the high dielectric material of the dielectric
resonator in conjunction with the configuration of the dielectric
resonators the microwave field is redistributed such that an
increased magnetic energy density (H.sub.1) is experienced in the
sample volume since in the TE.sub.01.delta. mode this resonator
will have a vertically directed H.sub.1 extending through its
center whereas the electric field (E.sub.1) is primarily confined
within the dielectric itself. This redistribution and separation of
the E.sub.1 and H.sub.1 components helps yield significantly larger
EPR signals and a greater Q for an aqueous sample.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it would be well
understood by those skilled in art that other changes on a
particular form and details may be made therein without departing
from the spirit and scope of the invention.
* * * * *